This document relates to configurations of electric machines, such as those with motors, generators and those that are operated as both a motor and a generator (collectively motor/generators).
A motor/generator sized to accelerate a load is typically much larger than needed to maintain the load rotating at steady state speed. Consequently, the motor/generator is not optimal for steady state operation and experiences losses, often called iron losses, stemming from the losses in the core of the stator. One type of electric machine where these types of losses are particularly relevant is a flywheel power storage system. In a flywheel power storage system a flywheel is driven by a motor/generator to store electric power in the form of kinetic energy. When the power is needed, the flywheel's kinetic energy is discharged in driving the motor/generator to generate electric power. Such flywheel power storage systems are typically used in UPS systems to store power for use in a power failure and/or power regulation system to supply power when the primary source of power is irregular or of poor quality. As such, the flywheel power storage system's motor/generator spends most of its operating life at or near steady state speed in standby or waiting mode. These iron losses are an operating cost that impact the cost effectiveness of operating a flywheel power storage system, and particularly so in very large flywheel applications where standby time is significant. In other applications of motor/generators like pump motors and turbine generators, these losses are present and are part of the total machine operating loss, affecting the operating efficiency of the machine. For example, iron losses as low as 1% in a 1 megawatt (MW) system can amount to a 10 (kilowatt) kW loss, the primary standby operating loss in a flywheel. Some motor/generators can minimize these losses by using field coils to provide a field for the motor/generator during high loads that are then shut-off or minimized in standby mode to reduce losses. These systems, though, come with the trade-off of lower output efficiency during operation, potential failure points in the field coil and its power supply, delay in output power while the field is increased when the coil is turned on, higher rotor heating, and larger size. These trade-offs are less of an issue for permanent magnet motor/generators because they have a constant field that is highly reliable and do not need field coils or an associated power supply. However, unlike a powered field coil, permanent magnets cannot be turned off to reduce losses during standby mode.